Embedded cant indicator for rifles

ABSTRACT

A device comprised of bubble level embedded into a solid block of material and located above the rifle grip and behind the barrel to facilitate the measurement and compensation of the rifle&#39;s cant while lining up the rifle&#39;s sights. The device is precision machined to allow the bubble to be aligned with the axis of the barrel to produce a true “zero-cant” condition. Graduation markings are placed on either left or right of the bubble level to allow the user to judge the relative degree of cant. The location allows cant adjustment without having to change the shooter&#39;s position.

CROSS-REFERENCES

This is a Divisional/Continuation-in-Part Application from U.S.nonprovisional application Ser. No. 15/061,613, which is a continuationof Ser. No. 14/842,925, which is a continuation of Ser. No. 14/154,214and claims all priority to the aforementioned applications.

FIELD OF THE INVENTION

This invention relates generally to rifle style firearms, andparticularly concerns both apparatus and methods for readily andprecisely determining the cant of the rifle relative to the target suchthat the shooter can make appropriate aim adjustments to improve theprobability of hitting the target. The location of the cant measurementdevice is designed to facilitate the shooters assessment of the rifle'scant when viewing other sighting mechanisms on the rifle.

BACKGROUND OF THE INVENTION

Accuracy in placing a projectile onto a target using a rifle requiresthe shooter to determine three primary elements: 1) distance to thetarget, 2) the incline of the projectile as it leaves the rifle, and 3)the cant of the rifle at the moment the projectile leaves the rifle. Arifle's cant is defined as the degree of rotational tilt the rifle hasalong the axis of the barrel. Distance and incline are part of“sighting” a rifle to a target and generally require several elementsthat must be determined by the shooter. Since gravity tends to bring theprojectile downward, an incline of the barrel must often be made to hita target at some distance. Commonly used sights for setting the inclinecan be a groove or aperture at the rear end of the post or the point atthe barrel end-muzzle. Once the shooter determines a point at which theprojectile is aimed, the shooter uses one eye to align the post into thegroove, which effectively aligns the rifle both horizontally andvertically to the point of aim.

However, such sighting mechanisms may not offer the shooter with thedegree of accuracy that may be desired. To improve the accuracy of thehorizontal and vertical alignment, some rifle assemblies make use of amagnification mechanism, referred to as a scope. A scope typicallyprovides the shooter with a glass view port displaying horizontal andvertical lines in addition to a magnified view of the point of aim.Scopes incorporate vertical and horizontal adjustment mechanism. Theshooter simply makes the calculated vertical and horizontal adjustmentsto account for situational issues such as wind, temperature, anddistance and aligns the point of aim with the intersection of these twolines, commonly referred to as cross-hairs. Scopes can contain a systemof lines, dots, cross hairs, wires, or electrically projected imageswhich aid aligning the barrel to the point of aim. Scopes are generallymounted on top of the action assembly near the back end of the barrel ofthe rifle and are attached thereto with some mechanism for adjustment. Acommon adjustment mechanism is a ring and slotted bar-rail device alsoknown as scope rings and scope bases. These adjustments are typicallymade at a shooting range or target practice area where the rifle isplaced in a holder to ensure proper alignment and target distances areaccurately known. Using this method, a rifle and scope can be adjustedto provide the shooter with a high degree of accuracy.

However, using a holder at a shooting range for calibrating a rifle'sproper incline as a function of distance to the target often does notrepresent real world situations where the shooter is either standing orprone with the rifle being held at the time of firing. In thesesituations, the rifle is often twisted or rotated about the axis of thebarrel. The physics of projectile firing is greatly affected by thisdegree of rotation or “cant” of the rifle at the moment of firing. Forexample, a left angle of cant tends to result in the projectileimpacting the target to the lower left of the point of aim. Shooters,especially competition target shooters, must compensate for the cant ofthe rifle to improve shot accuracy.

Various mechanism have been presented in the prior art to providefeedback to the shooter of the degree of cant during their aim. One suchexample is U.S. Pat. No. 6,813,855 where Pinkley presents an apparatuswhere among other accompanying pieces, a bubble level is strapped to therifle stock underneath the scope. Pinlkley's cant compensation methodinvolves the steps of positioning the firearm and scope with a cantedreticle system so that its vertical axis is positioned as indicated bythe level bubble of the mounted level sub-assembly, positioning thevertical reference shaft sub-assembly a distance from the muzzle end ofthe firearm. The shooter then rotates the scope on the firearmsufficiently to align the vertical cross hair of the scope reticlesystem with the distant vertical reference shaft sub-assembly. Lastly,the shooter locks the sighting scope in the corrected position on thefirearm.

The prior art attempts to provide the shooter with feedback for the cantof the rifle tend to be attached to the scope and as such areaccessories that must be carefully assembled to the scope and are notsuitable for shooting situations where speed and durability arerequired. Also, the prior art cant measurement systems themselves mustbe thoroughly tested and calibrated by the shooter so that typicallyonly that shooter, with that cant feedback device, on that specialrifle, and carefully calibrated by a trained technician can be used toproduce the degree of accuracy in critical or competitive shootingenvironments.

It has been discovered that by locating a pre-calibrated bubble levelbetween the shooter's eye and the scope and carefully machining thebubble level within the rifle system, a reliable cant feedback systemcan be readily made available to any shooter and repeatable across anentire weapon platform. Also, by carefully embedding the bubble levelwithin the body of the rifle, the cant feedback method can be durableand repeatable for a whole range of shooting applications, especiallyfor the war-fighter.

Additionally, by embedding the cant level indicator into the rifle'sstock and providing an accurate measurement of the rifle's cant,calibrating the firearm is greatly facilitated. By placing a plumb lineat the desired target calibration distance (100 yards or 100 meters, forexample), and then aligning the vertical reticle of the scope with therifle at zero cant, the scope reticle-aiming reference are “trued” tothe cant axis of the rifle. Once this initial process is completed, therifle and scope are now calibrated for a “zero-cant” condition relativeto each other.

SUMMARY OF THE INVENTION

The present invention presents an embedded precision level indicatorthat provides a true reference to level or plumb allowing for the finalcant correction to be made before the shot is taken. The presentinvention provides the rifle shooter with a tool that greatly enhances“first-round-hit” probabilities and increases overall accuracy. Theinvention is located so that the rifle shooter does not have to changeor disturb his body position to monitor the cant of the rifle.

The invention is machined into the rifle stock component and is alignedwith the horizontal axis of the center line. This horizontal axis isperpendicular to the center line referenced from top to bottom. Thisaxis can also be described as the 3 o'clock to 9 o'clock “cant axis”.This invention can now provide the rifle shooter with one morecalculation used in making the perfect shot at even longer distances tothe target. This feedback is of critical importance because cantmeasuring mechanisms of the prior art did not solve the issue of“man-introduced-errors” because the mounting of the cant level indicatoris often not performed by a specially trained technician. Thesespecially-trained technicians often included several items installed forreference points with none of them truly being calibrated to thehorizontal plane.

The invention presented herein is machined into the assembly on the sameplane as the cant axis. The accuracy of locating the invention true tothe cant axis is enhanced by the use of computer-aided tools with verysmall error tolerances. A set of cant reference gradients is machinedinto the assembly as well. The reference gradients allow for duplicatingthe cant if a “Zero Cant” condition is not achievable. The shooter canperform a quick calculation that formulates the amount of “Point of Aim”adjustment required to successfully engage the target due to the amountof Cant introduced into the rifle. This combination of location,precision machining and calibration feedback allows shooters with muchless experience and in situations of duress to greatly improve shotaccuracy.

The invention herein presents methods of using the embedded cantindicator to provide a shooter with an optimized and compensatedalignment system that maximizes the accuracy and repeatability ofhitting the shooter's target and provides a weapon system that achievesthis high level of precision compensation between the target and weaponin a most convenient manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the invention apparatus located in itsposition relative to the stock of the rifle and overall weapon platform.The barrel of the rifle and the sighting mechanisms are not shown butwould be understood to one skilled in the art where these components arelocated on this rifle stock assembly;

FIG. 2 top side view of the invention showing the bubble and calibrationmarks;

FIG. 3 is a left side view of the invention showing the variouscompartments housing the bubble level and interfacing with the remainedof the rifle components;

FIG. 4 is bottom view of one embodiment of the invention indicatinglength and width dimensions;

FIG. 5 is a top-down view of a cant indicator positioned between abolt-action receiver and a rifle's stock section; and

FIG. 6 is a side view of a firearm with a cant indicator having a lightsource to illuminate the bubble within the bubble level.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of the invention as an element of anoverall weapon assembly. This weapon assembly is comprised of multipleparts that are bolted together to provide a functioning rifle. In itsmost basic form, this particular weapon platform is comprised of a stock1, the cant indicator 20, the butt of the rifle 4, a recoil absorbingpad S. The barrel of the rifle attaches at point 2. If the shooterdesired a scope, it can be attached to the stock at 6. As the shooterholds the weapon and views a target through the scope in a directionparallel to the axis of the rifle barrel, the cant indication is beobtained by glancing downward at a bubble contained within the cantindicator assembly 20. By using the calibration lines, as will befurther described in later Figures, the position of the bubble to theleft or right of the center line will indicate the extent with to whichthe rifle is tilted about the axis of the barrel. A shooter can eitherrotate the weapon to eliminate the cant or make the appropriateadjustment to the point of aim at the target. As has been learned fromcompetition shooting, extreme accuracy of hitting the target typicallyrequires a precisely trained and practiced shooter and a custom-setrifle that is calibrated to the shooters particular method of holdingand aiming the firearm. Cant compensation is critical to improving shotaccuracy. The special relationship between the shooter and his weapon(i.e., knowing where the projectile will travel when aimed thisparticular way, in this particular environment, and on this particularweapon) is extremely important in high-accuracy shooting. Modernballistic software-based calculators also have a function that theshooter can input the cant offset and the appropriate aim correctionsare computed and displayed. The present invention allows the same levelof high-accuracy shooting because every cant indicator is preciselymachined and attached to the particular stock design which employs thedevice. Therefore, a given shooter can pick up any weapon with theinvention installed and the cant compensation will be precise andrepeatable. Now, a trained shooter has multiple weapons each with apredictable response to cant compensation. This eliminates the need fora shooter having only one rifle which they can use that is set up by aspecialist for high-accuracy shooting. In addition, more shooters can betrained because more weapons are available with high-accuracy cantcompensation systems.

FIG. 2 illustrates a top view of the section of the rifle containing theprecision-machined bubble-level system that comprises the invention. Inone embodiment of the invention, the device is machined out of a solidblock 20 (For example, as shown in FIG. 1 out of a component of thestock or the stock itself just behind the action receiver). Differentmaterials can be used depending on the level of cost and durabilitydesired. a cavity for receiving a bubble level is machined directly intothe stock section 20 just behind the action receiver relative to theshooter's position during firing. The dashed lines 21 indicate a tubularcavity that is machined out of the block through from one side but doesnot extend completely through to the other side. This allows the bubblelevel tube to be inserted or removed from one side and when pushed in,is held in proper place by contacting the closed end of the cavity. Thiscavity length is also precisely machined so that the bubble level wheninserted is correctly aligned to the gradient markings 29. The diameterof the cavity is machined with very close tolerances (0.001″ typically)to the diameter of the bubble-level inserted therein. The length andwidth of the cavity 21 are dictated by the diameter and length of theparticular bubble-level incorporated into the weapon system. In oneembodiment, the bubble level is 0.375″ in diameter and the length is1.500.″ An oval-shaped viewport 22 is machined into the top face thatallows the shooter to view the bubble level tube that is inserted intothe cavity. A bubble-level comprised of a glass or plastic tube sealedon both ends and containing a phosphorous liquid or other fluorescentliquid, but not completely filled, such that a bubble exists within thesealed tube. In this Figure, gradient markings 29 are spaced apartevenly along both the top edge and bottom edge of the viewport 22. Thesemarkings provide a reference point for the shooter to judge the level ofcant displayed by the movement of the bubble within the level either tothe left or right of true center. The markings are evenly spaced but areprimarily for consistent reference and do not represent any actualdegree of rotation. Once the shooter practices firing the weapon in acanted situation, the location of the bubble relative to the left orright count of the markings is sufficient for the shooter to be accuratein cant compensation in other shooting circumstances. The gradients canbe converted to actual degree of cant rotation simply by fieldcalibration. This may be desirable, for example, if the shooter is usinga ballistics software program that requires degree of rifle cant as aninput value to the calculation.

FIG. 3 illustrates a side view of one embodiment of the currentinvention showing various elements machined into the block 20 (In theembodiment shown in FIG. 3, into a removable piece of solid material 20that is inserted between the butt of the rifle and barrel/actionreceiver above the rifle's grip section). Items 23 and 24 are drilledand tapped to receive an attachment mechanism, such as machine screws,for attaching the invention to the section of the stock above the gripof the stock. In one embodiment, these machine screws are ¼″ diameter⅝″length and tapped to standard machine threads. All external surfaces ofthe invention are machined to flat to ensure that the device attachessecurely and is aligned with the other elements of the weapon that areattached to the bubble level insert. When properly placed on top of theweapon's grip section, these two screws point upward and secure thedevice to the grip section. In one embodiment items 26 and 27 aremachined and tapped cavities to receive four attachment mechanisms.These attachment mechanisms secure and align the device to the butt ofthe rifle. In one embodiment, the attachment mechanisms are 10/32″machine bolts. The invention also includes a contact base 25 thatextends down below the back end of the stock. This left side of thisextended surface provides a contact surface for attaching a foldinghinge piece or any other accessory item bolting on to the back end ofthe stock.

In one embodiment, the same principles for housing the bubble level canbe applied to raise the cant level indicator above the top surface ofthe stock. In one such embodiment, a rectangular bubble holder (or othershaped object or mass) is machined that protrudes upward from the topsurface of the stock, behind the receiver. In this embodiment, thebubble holder has a cavity within it that is aligned with the horizontalaxis of the barrel. As with other embodiments, the bubble level fitsinto the cavity so that the bubble level is aligned with the horizontalaxis of the barrel when it is inserted into the cavity. As with otherembodiments, the bubble holder also has a view port machined into thebubble holder so that a shooter can see the bubble inside the bubbleholder. In some embodiments, the view port is on the top of the bubbleholder. In other embodiments, the view port is on a rear-facing side ofthe bubble holder. In other embodiments, the view port is on both therear-facing and the front-facing side of the bubble holder so that lightwill pass all the way through the bubble level. The bubble holder can beany shape or size so long as it does not interfere with the shooter'sview of the sight reticle. In one embodiment, the bubble level issuspended in a frame extending at least partially above the top surfaceof the stock behind the receiver. In one embodiment, the frame isattached to the stock. In one embodiment, the frame is a unitary pieceof the stock (i.e., part of the stock) or is part of a larger componentof the stock. In an embodiment with a frame protruding at leastpartially from the top surface of the stock, the bubble level is heldinside the frame and the bubble level can be protected by the frameagainst being bumped or knocked out of alignment with the horizontalaxis of the barrel.

In another embodiment, the cant indicator can be machined directly intothe action receiver of a rifle. A firearm receiver has severalcomponents, including a bolt assembly and a receiver body. The boltassembly has a body, a handle, and a rear firing pin shroud. Thereceiver body is typically a tube-shaped support for the bolt assembly.Generally, a receiver body has a tang (rear facing end of the actionreceiver, often tapered and located below the firing pin shroud when theaction receiver is assembled) that extends from the back of the actionreceiver. In such an embodiment, a cant level indicator can be machineddirectly into the tang. In such an embodiment, a cavity and a view portare machined into the tang either from the side or from the top. Thecavity in such an embodiment is similar to other embodiments and holds abubble level within the tang. Of course, a smaller bubble lever andcavity may be required to fit into the tang. The view port allows ashooter to see the bubble level within the cavity. In some embodiments,the bubble level is only partially recessed into a top surface of thetang, and a view port is not required because part of the bubble levelis above the top surface of the tang.

In reference to FIG. 4, the bottom view of the invention in FIG. 3 showsthe two threaded holes 23 and 24. In addition to securing the inventionto the stock via these attachment mechanisms, two ovular sections 26 and27 are removed from the insert assembly for receiving similarly-shapedraised sections placed on top of the rifle's grip section. This “tongueand groove” style connection ensures that the invention is properlyaligned and located relative to other components that make up the weaponassembly. This connection also adds strength to the invention andweapon's grip section to better handle the stresses of firing the rifleand the weight of the stock components that are bolted to the bubblelevel insert. In one embodiment, the height and width of the bubblelevel insert is 1.75″ and 1.23″ respectively. However, it is understoodthat the dimensions of the invention are relative to the size of thebubble level device incorporated into the firearm.

In reference to FIG. 5, a cant indicator assembly 20 is shown positionedbetween a bolt-action receiver 30 and the shooter and is incorporatedinto the rifle's stock section 31. In this position, a shooter wouldhave the vantage point of viewing the bubble level 32 when gripping therifle in a firing position. In the event a sight reticle is alsoinstalled above the action receiver, the shooter can readily view thecant indicator 20 after viewing the sight reticle simply by glancing theeyes downward. Little to no head movement is required, which allows theshooter to more effectively concentrate on the sight reticle view andadjust the aim to account for any cant indicated by the bubble level. Atlonger and longer target distances, even subtle rifle alignment errorscan greatly magnify the projectile's flight path away from the desiredlocation. Consequently, a shooter will desire to perform the leastamount of physical movement when focusing their aim. This reduction inunnecessary movement includes controlling breathing, slowing the body'spulse, and minimizing head and body movements. By positioning the cantindicator 20 behind the action receiver, a shooter can readily make alast-second check and compensation for cant just before the rifle isfired at the target. To further enhance the shooter's accuracy incalibrating the rifle's cant/sight reticle compensation relationship,division lines 29 can be located either above or below the viewportcavity 22 that allow even more precise assessment of the bubble's offsetfrom true center. In another embodiment, the rifle's stock material canbe removed around the bubble level 32 to enlarge the viewport cavity 22.By removing more stock material and enlarging the viewport cavity 22around the cant indicator assembly 20, additional weight can be removedfrom the rifle, which may be desired by the shooter.

In reference to FIG. 6, a side view of the cant indicator assembly 20 isshown positioned above a firearm's grip section 33. A light-sourcecavity 34A is installed into the firearm's stock section near oradjacent to the bubble level 32 and extending below and away from thebubble level 32. The light source is placed within this cavity 34Abefore the bubble level 32 is inserted, so that the light projects upand illuminates the bubble level 32 when the bubble level 32 is inplace. The size and dimensions of the light-source cavity are determinedby the size of the light-source itself, or by preference. The luminosityof the light source may also vary depending on the preferences of theshooter. The light source may also be removable from the cavity orperiodically changed based on the preference of the shooter or if thelight source needs to be recharged or replaced. In one embodiment, thelight-source cavity may be larger than the light source to allow thelight to radiate in several directions. When the bubble level isinserted into its cavity, the light-source is also secured within itscavity. By controlling the lumens or location of the light source, ashooter can observe the rifle's cant in dark environments. In oneembodiment, the light source is positioned on a rifle so that it cannotbe seen by anyone other than the shooter so as not to give away hisposition. For example, if the light source is above the top surface ofthe stock section, that light could also be observed and detected fromthe side of the rifle. Phosphorous-containing fluorescent liquids can becharged by an external light source but relatively quickly dissipatethat fluorescent charge when removed to a dark environment. Materialsuch as tritium, a radioactive isotope of hydrogen, naturally fluorescesas it decays and has long been used as a radioluminescent light sourcefor watches and instruments. A tritium light source, for example, couldbe an ideal material for military firearms because it can last thelifetime of the firearm and requires virtually no maintenance. However,any other light source or bubble level type could be used. In oneembodiment, the light source cavity 34A is located below the bubblelevel so that the light source projects upward toward the viewport 22 tobacklight the bubble level from the shooter perspective. However, thelight source can generally be located at other points around the bubblelevel, such as in front of the bubble level 34B (9 o'clock position) orbehind the bubble level 34C. In one embodiment, both the light sourceand the bubble level are embedded below the upper surface of the stocksection for protection. However, in another embodiment, the light sourceis fully embedded within the stock section but the bubble level is onlypartly embedded within the stock section. In another embodiment, thebubble level and light source cavities are incorporated into the rearend of the action receiver closest to the shooter instead of within theforward end of the stock section. In another embodiment, a combinationof phosphorous-based luminescent fluid is placed within the bubble level32 and tritium is used as the light source inside the light sourcecavity such that the tritium causes continuous luminescence of the fluidwithin bubble level. In addition to using of the illuminated bubblelevel in dusk and twilight situations, the light source can also makethe bubble level more visible in the daytime where external solar lightis limited by cloud cover or the shooter's particular firing environmentis intentionally darkened in some way to obscure his location. In anembodiment with a bubble level in the tang, the light source can bepositioned below, in front of, or behind the bubble level as with otherembodiments. In some embodiments, the light source is in the stock nearthe tang.

Although generally bubble levels are elongated tubular chambersincompletely filled with a liquid, various other changes may be made tothe apparatus in size, proportions, and material of construction toaccommodate other bubble level chamber designs without departing fromthe meaning, scope, or intent of the claims which follow.

What I claim for Letters Patent is:
 1. A method of compensating for thecant of a rifle comprising: a) Identifying a target through a rifle, therifle comprising a stock section, an action receiver section, a barrelhaving a centerline axis, a sight reticle and a cant measuring devicefurther comprising a cavity positioned within the rifle's stock sectionbehind the action receiver section having a front wall, and left andright side walls, a tubular bubble level placed into the cavity suchthat the bubble is closely aligned with the centerline axis of thebarrel when the rifle is at zero cant and marking lines for observingthe relative position of the bubble as the rifle is rotated about thebarrel's centerline axis, b) Aligning the sight reticle with the target;c) Assessing the degree of cant to the rifle by observing the left orright displacement of the bubble relative to bubble level's positionmarking lines; and d) Rotating the rifle about the axis of the barrelsuch that the bubble inside the level aligns with the centerline axis ofthe barrel.
 2. The method of claim 1 further comprising the step ofrealigning the sight reticle cross-hairs relative to the target afterremoving the cant from the rifle.
 3. A method of compensating for thecant of a rifle having a barrel, a stock section, and a bubble levelembedded into the stock section, the center of the bubble level in linewith the axis of the barrel, the method comprising: a) Identifying atarget and its proximate distance from the rifle; b) Aligning the riflewith the target; c) Assessing the degree of cant to the rifle using thealignment of the bubble relative to the centerline axis of the barrel;and d) Rotating the rifle about the axis of the barrel such that thebubble in the level aligns with the centerline axis of the barrel.
 4. Acant measuring device for a rifle stock section, the stock sectionhaving a forward end for attaching to an action receiver and barrelfurther comprising: a) a cavity bored within the forward end of thestock having at least one sidewall and a centerline that intersects thevertical plane of and is perpendicular to the centerline axis of thebarrel; b) a topside of the stock having a view port created by removingmaterial from the stock such that the cavity can be viewed when therifle is held in a shooting position; c) a bubble level inserted intosaid cavity through a circular hole bored through one sidewall and thehole having a diameter equal to or larger than the diameter of thetubular bubble level's diameter such that at zero cant, the bubble levelis aligned with the barrel's centerline axis.
 5. A cant measuring devicefor a rifle stock comprising: a stock having a top surface, a side, anda forward end, the forward end for attaching to an action receiver and abarrel; a cavity in the forward end of the stock and recessed below thetop surface of the stock, further comprising: a first end open to theside of the stock, a bubble level inserted in the cavity, and a viewport in the top surface of the stock for viewing the bubble level.
 6. Acant measuring device for a rifle stock comprising: a body having a topsurface, a side, and a forward end, the forward end for attaching to anaction receiver and a barrel; a cavity in the forward end of the bodyand recessed below the top surface of the body, further comprising: abubble level inserted in the cavity, and a view port in the top surfaceof the body for viewing the bubble level.
 7. The device of claim 6,further comprising the bubble level oriented parallel to a horizontalaxis of an attached barrel at zero degrees of cant.
 8. The device ofclaim 6 further comprising the bubble oriented perpendicular to thevertical centerline of the body.
 9. A method of machining a cantmeasuring device for a firearm comprising: a) determining a cant axis ofa stock; b) machining a cavity below a top surface of the stock toreceive a bubble level, the cavity being perpendicular to the cant axisof the rifle and being larger than a bubble level; c) machining a viewport in the top surface of the stock to view the cavity; and d)embedding the bubble level within the cavity; wherein a bubble iscentered in the bubble level when the stock is at zero degrees of canton the cant axis and the bubble can be viewed through the view port onthe top surface of the stock.